This invention relates generally to processing audio signals and, in particular, to removing an undesirable periodic noise component from an audio signal without perceptibly affecting the remaining signal.
It is common in musical instruments, particularly with electrically amplified guitars, to sense string vibration with a magnetic pickup. Typically, the sensor or pickup includes of a series of magnetic cylinders located concentrically within a wire winding. As the metallic strings of the guitar vibrate close to the magnetic cylinders, a varying electric field is generated around the magnetic cylinders. This varying electric field causes a current to flow through the wire winding that is proportional to the displacement of the strings from the magnetic cylinders. The current is passed to an amplifier system in which the signal is amplified, processed, and ultimately converted back into an acoustical signal by a loudspeaker or a headphone.
The purpose of the wire winding is to detect movement of the strings. However, the winding is sensitive to electromagnetic signals from any source, not just the string/magnet system. The most common sources of electromagnetic signals include power conversion circuitry, such as dimmers in lighting systems, transformers, cathode ray tubes (CRTs) as used in home computer systems and television sets, and digital clock signals used in appliances containing a microcontroller. This electromagnetic interference (EMI) imposes an undesirable xe2x80x9chumxe2x80x9d in the musical instrument signal, wherein each source produces a characteristic signal.
Various methods have been tried for reducing the effect of EMI on magnetic pickups. For example, a xe2x80x9chumbuckerxe2x80x9d pickup utilizes two pickups as described above in close proximity to one another, but having oppositely wound wire windings. If the EMI present within the two windings is equal, the opposed windings will cause the currents induced in the pickups to have equal amplitudes but opposite polarity. Summing the induced currents cancels the noise signal. Hum cancellation by this pickup is excellent in most conditions but the subjective qualities of the string signal are changed significantly relative to a single coil pickup.
Another device that has been used to reduce hum is the noise gate; e.g see U.S. Pat. No. 3,989,897 (Carver). The noise gate is like a squelch circuit wherein a signal from the pickup is reduced when its energy level falls below a specified threshold. If this threshold is set correctly, the undesirable EMI induced signal is reduced or eliminated in the absence of a signal from the strings. However, this method can result in reduction or elimination of the signal from the strings when its energy level is low. This effect can be more annoying than the noise signal itself.
Notch filters have also been used to try to eliminate hum. A notch filter attenuates a signal in a very narrow band of frequencies. Digital implementations of notch filters make it relatively easy to provide a plurality of notches, at the fundamental frequency of the hum and its harmonics, whereby the hum can be reduced or eliminated completely.
As noted above, the various sources of EMI each have a characteristic signal. Depending on the source, many notches may be required in the notch filter to reduce the hum signal satisfactorily. This is especially the case for impulse noise, such as emitted by CRTs. Impulse noise contains significant high frequency energy. To be effective on these signals, a notch filter requires either extensive hardware or a powerful digital signal processor (DSP), making the implementation expensive and complex. Also, a notch filter changes the subjective quality of the signal of interest by changing its tonal characteristics and causing xe2x80x9cdropoutsxe2x80x9d for notes, or their harmonics, at or near the center frequency of a notch.
U.S. Pat. No. 4,733,193 (Klokocka) discloses a system in which an input signal is divided between two channels, one of which goes directly to a subtraction circuit. The other channel includes a delay loop for enhancing the hum signal, which is then coupled as another input to the subtraction circuit. The gain of the loop and the gain of the channel vary inversely to try to cancel the hum.
Several proposed methods for eliminating periodic noise from an audio signal require complex digital and analog circuitry that is expensive to implement.
These prior art designs tend to be ineffective in eliminating a periodic noise signal, excessively disruptive to the signal of interest, exceedingly complex and expensive, or a combination of these faults.
In view of the foregoing, it is therefore an object of the invention to provide a technique for eliminating a periodic noise with minimal disruption of the signal of interest.
Another object of the invention is to provide a technique that can be implemented in hardware or software at relatively low cost.
A further object of the invention is to provide a technique for eliminating periodic noise from a signal wherein the technique is easily adapted to any one source of noise or to several sources of noise.
The foregoing objects are achieved by this invention wherein an input signal is applied to a notch filter having a transfer function that is the inverse of the expected noise signal. The filtered signal is coupled to a first amplifier and the input signal is coupled to a second amplifier. The outputs of the amplifiers are summed. The gains of the amplifiers are oppositely adjusted in response to the magnitude of the input signal. At low amplitude, the filtered signal is amplified more than the unfiltered signal. At high amplitude, the unfiltered signal is amplified more than the filtered signal.